(a) Field of the Invention
The present invention relates to distributed-feedback semiconductor laser device (referred to as DFB laser device hereinafter) having ununiform arrangement of a diffraction grating.
(b) Description of the Related Art
A DFB laser device includes a diffraction grating having a periodic structure of the refractive index, wherein the real part and/or imaginary part of the refractive index changes periodically along the axial direction of the laser cavity. The diffraction grating allows a specific wavelength of the laser emission to be fed-back to the laser cavity, whereby the DFB laser device has a higher wavelength selectivity and thus lases at a single mode, i.e., at a signal wavelength.
Due to the higher wavelength selectivity as described above, the DFB laser device is suitably used as a light source for an optical communication. The DFB laser devices used in the optical communications are categorized in a variety of types including continuous wave emission type (CW-DFB) and, direct modulation type (DM-DFB).
A high-output-power CW-DFB laser device is generally used in combination with an external optical modulator and is used in an analog signal transmission. The CW-DFB laser device has an asymmetric reflection structure, wherein a low-reflection coating film is formed on the front facet (emission facet) of the laser cavity and a high-reflection coating film is formed on the rear facet. This asymmetric reflectance structure allows the laser cavity to emit most of the laser power at the front facet, thereby providing a higher output power efficiency.
In the CW-DFB laser device, the coupling factor κ is optimized in consideration of the single-mode characteristic, output power efficiency etc. in general. More specifically, an excessively higher κ value impairs a high output power, whereas an extremely lower κ value prevents the single-mode emission. Thus, the coupling factor κ is designed within a suitable range, and it is considered that κL, i.e, product of the coupling factor κ by the cavity length L, which is around “1” is most suitable from the view points of the single-mode characteristic, output power efficiency etc. The coupling factor κ in a DFB laser device is designed at a specified value by adjusting a variety of parameters such as the layer thickness, composition or duty ratio of the diffraction grating and the distance between the diffraction grating and the active layer. The term “duty ratio” as used herein means the ratio of the length of a higher refractive index portion to the length of a lower refractive index portion in the diffraction grating, the lengths being measured along the axial direction of the laser cavity.
In addition, in order to obtain a higher output power, it is effective to design a larger value for the cavity length L. The larger cavity length provides a higher output power while suppressing the thermal saturation, which generally occurs due to the heat generated by a higher driving current. To employ a larger cavity length L, however, the coupling factor κ should be designed smaller in view of the desired relationship κL 1.
More specifically, if an extremely large cavity length L, e.g., L=1000 μm is desired, an extremely small coupling factor κ 10 cm−1 should be employed in view of the relationship κL 1. It is difficult in fact to achieve such a low coupling factor, however. It may be considered to employ a smaller layer thickness of the diffraction grating, or a smaller difference between the refractive indexes of the diffraction grating and the buried layer buried the diffraction grating, in order to achieve such an extremely low coupling factor. The mentioned smaller thickness or the smaller difference requires a higher accuracy in the fabrication process of the laser device, and is generally difficult to employ in the mass production of the laser devices. Thus, it is difficult to fabricate DFB laser devices having excellent single-mode characteristic in the conventional technique with a higher product yield.
For achieving a low coupling factor, it is possible in the conventional technique to employ a partial diffraction grating, wherein the diffraction grating is formed for a part of the laser cavity. The partial diffraction grating can advantageously reduce the effective coupling factor to the extent determined by the ratio of the length of the partial diffraction grating to the cavity length. However, this structure causes a discontinuity of the electric field at the boundary between the area including the diffraction grating and the area including no diffraction grating, raising the problem of ununiformity of the carrier density in the injected current. Thus, the gain difference between the lasing modes may be reduced, which causes an inter-mode transition in the laser device at a higher output power range, thereby generating an undesirable kink in the current dependency characteristic of the optical output power.
In the meantime, a conventional DFB laser device having a low-reflection coating film formed at the front facet of the laser cavity and a high-reflection coating film formed at the rear facet, as described before, generally involves a problem of hole burning, which causes an increase of the spectral linewidth in the laser emission.